Forecasters May Be All Wet When It Comes To Summer Storms

July 11, 1993|By Michael A. Lev.

He studies the high-tech weather radar and satellite gear, and consults the predictions spit out by a supercomputer at headquarters in Washington. But when Allan Morrison of the National Weather Service writes the afternoon forecast for Illinois this day, only his meteorological Muse knows for sure.

In a burst of inspiration, he lowers the chance of stormy evening weather in Chicago from 40 percent to 30 percent.

"I'm hedging my bets," he explained from his computer terminal in the Weather Service's Romeoville office on Thursday. "I wouldn't be a bit surprised if nothing happens."

Meteorologists like Morrison are feeling some high pressure this summer. Those who usually predict "rain showers" one day and "partly sunny" the next so authoritatively are practically writing their forecasts by throws of the dart.

Sure, most people figure TV weather forecasters do that anyway. But the fact is that the computer models and other scientific data from the National Weather Service-the backbone of most forecasts-usually get it right.

Except in a summer like this.

With wave after wave of summer thundershowers rolling through the Chicago area, Doppler radar starts to look like a very expensive Ouija board and weathermen curse the skies.

The problem is that thunderstorms, like lightning bugs, are randomly appearing creatures. Sometimes they organize into masses, like the waves of rainstorms that have ravaged the upper Midwest and flooded the Mississippi River. But often they are swiftly moving loners, perhaps only 5 miles wide, that dart from the heavens and set out on courses unknown.

The storms that appear most often in late spring and summer are created by weather systems that form along the boundary of warmer and cooler air. But because that border can shift from day to day, and because the conditions required for a severe thundershower are so exacting, weathermen say they often are at a loss to describe or predict a thunderstorm until it appears.

"It's virtually impossible to pinpoint exactly where a random scattering of thunderstorms will be, although you can be confident that they will be in the metro area," said Jim Candor, a meteorologist with Accu-Weather, the private forecasting company that supplies weather reports to many local radio and television stations.

That makes for confusing forecasts, because on any given day, a thin band of the Chicago area might get battered and drowned while the rest of the city enjoys a blissful sunning.

"Makes you crazy," said Steve Baskerville, Channel 2's weather forecaster, conceding that he feels guilty when he hangs the symbol for thunderstorm on practically every day of his five-day forecast, knowing full well many viewers might never get a drop of rain.

There isn't an alternative, however, because the cardinal sin of weather reporting is to promise sun and deliver hail.

Allowing a peek inside the forecaster's bag of tricks, Baskerville added, "People are much more pleased if you said it was going to rain and it was all right."

Paul Dailey, the meteorologist in charge of the National Weather Service office in Romeoville, said the agency typically gets about 90 percent of its forecasts right. But when it comes to predicting thunderstorms, he only gives himself a 70 percent accuracy rate.

"I think we've done a pretty good job, but I know we've made some mistakes," Dailey said.

That's even true since last month, when the office got its new Doppler radar, which can pick up details of wind and rain clouds that have not been available before. While that helps to track storms once they appear, it can't predict future ones.

That leaves forecasters vulnerable to mistakes like the one that occurred Monday, June 28th, when a wicked thunderstorm surprised the city and its weather people in the early morning hours. Dailey said forecasters missed the storm because it formed along a frontal boundary that sagged unexpectedly into the area.

"It just takes minor waves along the front to make it move 50 miles," he said. "We can be off by 50 miles on storm activity this time of year very easily, and 50 miles makes all the difference."

If forecasters are unsure about the daily risk of a thunderstorm, they also don't know for certain why the Mississippi River basin is flooding and the Chicago area, just to the east, is not.

While there is disagreement, many weather forecasters attribute the Midwest's problems to a combination of El Nin o-the warm Pacific current that adds moisture to the atmosphere-and a strong Bermuda high pressure center stuck in the Southeast that is blocking traffic behind it.

The stage was set for flooding when several late winter and early spring storms saturated the earth. Then came the unusual Bermuda high in June, which stalled storms that later regenerated and soaked western Illinois, Iowa, Wisconsin, Minnesota and other parts of the Midwest.

The same Bermuda high is causing the East Coast to bake in 100-degree temperatures.